This invention relates to electrostatography. More particularly, this invention relates to corona generating devices for applying electrostatic charge onto a suitable surface.
The basic electrostatographic process is disclosed in U.S. Pat. No. 2,297,691. In this process an electrostatographic plate comprising a photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity which reaches them and thereby creates an electrostatic latent image on or in the plate coating which may then be developed into visible form by applying a developer material, e.g., a powder, to the plate using any one of a number of development techniques generally known and used in the art. The developer material electrostatically clings to the plate in a visual pattern corresponding to the electrostatic image. Thereafter the developed image is usually transferred from the plate to a support material such as paper to which it may be fixed by any suitable means thereby forming a permanent print.
Instead of being developed by means of a powder, the electrostatic latent image may be developed using liquid development techniques such as those described in U.S. Pat. No. 3,084,043, for example.
The charging of the electrostatographic plate in preparation for the exposure step can be accomplished by means of a corona generating device whereby electrostatic charge is applied to the electrostatographic plate to raise it to a potential of approximately 500 to 600 volts. One form of corona generating device for this purpose is disclosed in the U.S. Pat. No. 2,777,957 wherein a plurality of parallel wires are connected in series to a high voltage source and are supported in a conductive shield that is arranged in closely spaced relation to the surface to be charged. When the wires are energized, corona is generated along the surface of the wires and ions are caused to be deposited on the adjacent photoconductive surface. Suitable means are usually provided to effect relative movement of the surface to be charged and the corona generating device. Such a device may have a single corona wire.
It has heretofore been established that consistent high quality reproductions can best be obtained when uniform potential is applied to the electrostatographic plate in preparation of the plate for exposure step. If the electrostatographic plate is not charged to a sufficient potential, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of developer material thereon will be correspondingly small. If, however, the electrostatographic plate is overcharged, the converse will occur and if overcharged sufficiently the photoconductive layer of the electrostatographic plate can be permanently damaged. The charging of an electrostatographic plate to a uniform potential is generally loosely referred to in the art as uniformly charging the plate and for convenience this expression is used hereinafter to refer to charging such a plate to a uniform potential.
Since the contrast value, comparable to the contrast values obtained from silver halide papers, of the electrostatic latent image may be related directly to the potential charge on the electrostatographic plate before exposure, it is apparent that if the plate is not uniformly charged over its entire area, the contrast value of the electrostatic latent image obtained upon exposure will vary in different areas on the plate, and an uneven or mottled effect will be visible on the image when developed.
A more uniform and controlled charge can be obtained by placing a biased wire screen between the corona wires and the electrostatographic plate. This screen, which may be insulated from or electrically connected to the shield, permits energizing the corona wires to a potential well above the corona threshold potential thereof without causing damage to the electrostatographic plate because the excess of corona current over that required for proper charging of the plate is drained off by the biased screen. This type of corona generating device is referred to in the art as a "scorotron".
While a corona generating device as described above produces a very uniform and controlled charge, a substantial proportion of the corona generated is consumed by the metal shield resulting in relatively high ozone levels. Also, any lack of rigidity of the screen leading to it being distorted will result in uneven charge distribution on the plate.